I just put 1K miles on mine this weekend with my GPS along for the ride. The speedo is about 10% off across the range. At 75 mph, for example, the GPS indicates actual speed of 66.5
I hate it but that's typical for all bikes except Harleys.
There are two ways to fix it. The easiest is to find a gear drive reduction like hot rod car builders use to mate a Chevy trans to a Ford engine.
Another option which I've seen used on other bikes is to permenantly modify the magnetic field of the speedo's ring magnet. This can be done using a magnet from an old hard drive (very powerful) but it requires extreme skill to not over do it.
I've been searching for the exact gear drive converter but have not yet found the right combination of thread size and reduction.
